Composite synthetic strings are already known. FR-A-2 491 098 discloses synthetic string having two components: polyurethane and multifilament threads of polyamide, the multifilaments being embedded in a polyurethane matrix. The polyurethane binder has breaking elongation that is much greater than that of the polyamide, and in conjunction with a special structure for the string it makes it possible to obtain high breaking strength with medium hardness, which gives rise to the string returning quickly to its initial position after the impact of a ball.
Also, there have appeared tennis rackets with a rigid frame made using an appropriate choice of materials (carbon, boron, etc. . . . ), and above all by using much thicker sections. In such rackets, the frame hardly deforms at all under the impact of a ball, unlike what used to happen. It is thus the stringing that has to absorb all of the force due to the impact of the ball. Under such conditions, the strings move less, and there is considerably less wear by abrasion due to the strings rubbing against one another, particularly during "lift" shots during which the main strings move over the cross strings. However, a denting phenomenon develops and becomes predominant, i.e. a phenomenon of localized wear which gives rise to dents appearing in the cross-strings, which dents decrease the section of the string very quickly and cause it to break. This phenomenon increases with increasing string hardness.
Thus, when such frames are used with monofilament strings, players say that the stringing "explodes". In fact, what happens is that a set of strings break after they have been dented as the result of friction between the strings.
The same phenomenon exists, though to a lesser extent, with string of the type described in the above patent. That string has an outside surface of polyurethane which is not as hard as the monofilament threads, and is consequently better at withstanding denting than are strings based on monofilaments.